Self-discharge is a problem of electrochemical cells, particularly for less expensive cells made by printing at least some of the active layers. The unwanted electrochemical activity occurs through an electrolyte layer connecting two electrode layers (i.e., an anode layer and a cathode layer). The electrolyte layer provides a pathway for ionic conduction between the electrodes. However, some electrical conductivity can also occur within the electrolyte layer, which short circuits the cells and supports the unwanted discharge.
Chemical and physical interactions of the electrolyte layer with its surroundings can also degrade electrochemical cell performance. For example, materials within the electrolyte layer can interact with one or more of the electrode layers to chemically alter the electrode layers. Evaporation of liquids from the electrolyte layer can significantly degrade the ionic conductivity of the electrolyte layer.
Coin cells are popular as low-power sources for voice-chips, such as those used in greeting cards, as well as for electrochromic or liquid crystal displays. However, such coin cells are quite bulky for use with printed products and require physical connections that add to their bulk and diminish their reliability. Even though the coin cells are sealed to protect their active layers, self-discharge is still a problem, especially when the cells are stored in hot humid conditions.
U.S. Pat. No. 3,230,115 to Tamminen discloses an early example of a printed battery. Printed side-by-side on a plastic sheet are pairs of electrodes overlapped by an electrolyte layer made of a viscous adhesive gel. Conductive inks are printed in contact with outer edges of the electrodes for connecting like electrodes to each other (for forming a multi-cell battery) or to an electrical load. A plastic adhesive paint is sprayed over the electrolyte layer to prevent evaporation of moisture while venting gas byproducts.
A battery laminate disclosed in U.S. Pat. No. 5,350,645 to Lake et al. features electrolyte and electrode layers printed on or otherwise applied to sheets that are stacked together to form batteries. U.S. Pat. No. 5,652,043 to Nitzan treats the problem of liquid evaporation from the electrolyte layer of a similar stacked electrochemical cell laminate by adding a deliquescent material to the electrolyte layer.
Although various steps are taken by Tamminen, Lake et al., and Nitzan to prevent deterioration of electrolyte, their printed electrochemical cells are still subject to discharge through electrolyte layers connecting electrodes. Since the capacities of printed electrochemical cells are generally limited, such self-discharging can significantly limit their shelf life.
Regardless of cell shape, confining liquid electrolyte has long been a problem. U.S. Pat. No. 5,225,291 to Rao solves this problem in marine batteries by using surrounding sea water as an electrolyte. Electrode plates are mounted on opposite sides of a dielectric plate. Activation of the cell is deferred until the plate assembly is immersed in sea water.